All that is true. I would drool at having Ellis or Parayko on that blue-line; that would make the surprising decent Leafs defense absolutely rock-solid and ready for a deep playoff run, but yeah, Kapanen would be the sweetener in such a deal for either of those guys; they are true top pairing guys. For Pesce it is probably more of an add to Kapanen instead of Kappy being the add.
Small edit to your comment: it s 22 points for Kapenen now. No, I am not sold on him somehow being a superstar-winger, but at the very least he looks like a 20 goal 50 point 2nd liner who can play PK time (and he really should be since he is a former 1st round pick), which has value, but no, certainly not 1-for-1 with a top pairing D on a non-toxic contract. For example, I don't imagine many Leafs fans would think Kappy alone gets a guy like Paryko; that would be a very lopsided trade.
For a Toronto/Carolina trade involving Kapanen, which is the rumor here, I can't imagine Toronto would be interested in anything other than a defenseman who slots into the top-4, particularly the right side because right now that is literally the only thing Toronto needs, so the piece going to the Leafs in a trade scenario is probably Pesce or Faulk.